I have never seen the Tour but there are a lot of ways to fix a dress so it won´t get in the way while doing a cartwheel, so I don´t see why they would skip a dress because of that. I´ve done cartwheels and even backflips in many different dresses and it has always worked out fine. It takes a little work on the dress though, maybe they decided they did´nt have the time for that. Wich is really a pity, because a cartwheel in a dress suited for that looks so nice!